1. Field of the Invention
The present invention relates to a formed article having cordierite as a main component and a method for manufacturing the formed article.
2. Description of Related Art
As a formed article having cordierite as a main component, an article of cordierite (a cordierite article) formed in a honeycomb structure is well known. This cordierite article is, for example, used as a catalytic carrier which holds catalyst in a particulate filter for purifying exhaust gas of an internal combustion engine disposed in a vehicle. This catalytic carrier is required to be light in weight and to have a characteristic for giving a low pressure loss to the exhaust gas. Therefore, thinner cell walls of the catalytic carrier formed in a honeycomb structure have been recently used.
However, as the wall thickness of the catalytic carrier is reduced, the mechanical strength of the honeycomb structure is lowered. Therefore, it is required that the cordierite article has high mechanical strength. Published Japanese Patent First Publication No. H11-309380 discloses a method for forming an article of cordierite in a honeycomb structure so as to have high mechanical strength. In this formation, the generation of pores (or pits) in cordierite formed in the amorphous phase is suppressed. Therefore, the density of the cordierite is heightened, and the mechanical strength of the cordierite article is heightened.
However, in this method, porosity of the cordierite article is reduced to 20% or less to heighten the mechanical strength. Therefore, when the cordierite article is used as a catalytic carrier receiving exhaust gas of an engine, the catalytic carrier is inferior in performance for purifying exhaust gas. Further, as the porosity is decreased, thermal expansion in the cordierite article is increased. When the cordierite article having a high thermal expansion is used as the catalytic carrier and the temperature of the catalytic carrier is considerably changed during the operation of the engine, large thermal stresses are generated in the catalytic carrier. In this case, cracks are sometimes generated in the catalytic carrier, or the catalytic carrier is sometimes broken.
Moreover, as the mechanical strength in a formed article of cordierite is increased, the cordierite article can be more stably used as a catalytic carrier.
Therefore, it is required that a formed article of cordierite has low thermal expansion and higher mechanical strength with adequate porosity.